(1) Field of the Invention
The present invention relates to the treatment of fluids containing suspended particulate matter and particularly to improvements in mixing, thickening and similar processes performed on such fluids. More specifically, this invention is directed to improved mixers, thickeners and similar devices and especially to apparatus for preventing clogging and leaking and minimizing abrasion damage of devices which receive streams of liquid containing suspended particulate matter. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The present invention is particularly well-suited for use in the mixing, thickening and the like of fluids which contain particulate matter. The operation of mixers, thickeners, columns and similar equipment is frequently accompanied by undesirable occurrences such as clogging and leaking and by erosion as a result of the abrasion action of the particulate material. These undesirable occurrences become more pronounced when it is desired to extract a fluid having higher than a normal or average particle concentration from the apparatus.
As an example, if the discharge from a mixer, thickener or similar device is required to have a higher particle concentration than the average level present within the device itself, a sedimentation process must occur within the device. However, that sedimentation process must be controlled in such a manner that deposits of particulate matter do not collect at the bottom of the vessel comprising the device and clog the discharge or outlet for the fluid having the high particle concentration. Because of the problems associated with such clogging, the prior art has resorted to the use of concentrating devices wherein the contents of the vessel were fluidized. Such concentrating devices may operate by means of gravitational sedimentation, by means of centrifugal force in a centrifuge or hydrocyclone, or through the use of appropriate filtration equipment.
Thickeners without agitators, and having a spillway for the excess or clarified fluid, are known in the art. Such thickeners are characterized by a simple hollow-cone discharge or outlet at the base of their vessels and must be carefully designed to match the environment of use so that a marginal concentration of particulate matter in the spillway flow is not exceeded and excessive deviations in concentration within the vessel are avoided.
In the case of prior art mixers or thickeners which include agitators, the degree of concentration of the particulate containing fluid which may be obtained is severely limited because the cross-section of the base of the vessel comprising the mixer or thickener becomes smaller in the direction of flow. Accordingly, there is the danger that the capacity of the discharge or outlet system of the mixer or thickener will be insufficient or that clogging in the base region will occur. In either case the particle concentration within the container may increase above the desired concentration. Accordingly, it has been common practice in the prior art to provide mixers and thickeners with movable mechanical elements to prevents clogging or the like. Such mechanical clogging prevention means may take the form of a screw-type conveyor located in the conical base portion or at the base of the discharge or outlet of the vessel of the mixer or thickener.
The above-briefly discussed problems associated with clogging and the like in mixers and thickeners have also been addressed by retro-fitting existing equipment with concentrators of the types briefly discussed above, namely centrifuges, hydrocyclones and filters with associated pumps. In the case of such retro-fitting, the fluid containing the particulate matter is fed by the pump to the centrifuge, hydrocyclone or filter which is physically located outside of or external to the mixer, thickener, column or similar device. Separation of the fluid into a particle enriched phase and a filtrate will occur in the concentrator and at least a part of the filtrate is re-introduced as a reverse flow in the concentrator.
It is to be noted that it has been proposed to solve some of the above-discussed problems by creating a turbulent flow around the longitudinal axis of the vessel of a mixer within a conical base section thereof. On this point reference may be had to published Japanese Application No. J 5 7071-628.